The present invention relates to an optical disk and an optical disk drive which are capable of super-resolution reproduction.
Optical disk memories, which accomplish information reproduction alone or information recording and reproduction by irradiation of a light beam, have been put to practical use as high capacity, fast-access and portable storage media for various files, such as audio data, image data and computer data, and it is expected that their development will continue. Techniques for increasing the density of optical disks include shortening the wavelength of a gas laser for cutting a master, shortening the wavelength of a semiconductor laser as an operating light source, increasing the numerical aperture of an objective lens and making optical disks thinner. Further, with regard to recordable optical disks, various approaches such as mark length recording and land/groove recording are possible.
As a technique having a great effect on improvement on the density of an optical disk, a super-resolution reproduction technique utilizing a medium film has been proposed. The super-resolution reproduction technique was originally proposed as a technique specific to a magneto-optical disk. According to the super-resolution reproduction technique for the magneto-optical disk, a magnetic film (a super-resolution film) is disposed on the reproduction-beam incident side with respect to a recording layer, and both are exchange-coupled or magnetostatically coupled. Then, the temperature of the super-resolution film is raised by irradiation of a reproduction beam to change exchange the force or magnetostatic force, thereby forming an optical mask or an optical aperture in the super-resolution film to realize super-resolution reproduction.
Later, attempts were made in a ROM disk other than the magneto-optical disk, to provide a super-resolution film, whose transmittance varies with irradiation of a reproduction beam, on the reproduction-beam incident side with respect to a recording layer in order to perform super-resolution reproduction. Thus, it has been revealed that the super-resolution reproduction techniques can be adapted to all optical disks such as a magneto-optical disk, a CD-ROM, a CD-R, a WORM and a phase-change optical disk.
The super-resolution reproduction techniques can be classified into a heat mode and a photon mode. Examples of the conventional super-resolution films will be explained.
In the heat mode, a phase change material is employed as a super-resolution film, and the super-resolution film is heated by irradiation of a reproduction beam so as to cause phase change, thereby forming an optical aperture smaller than the reproduction beam spot. The shape of the optical aperture conforms with the isothermal line of the super-resolution film. Since the size of the optical aperture varies easily depending on the ambient temperature, it requires severe thermal control of the super-resolution film in accordance with the linear velocity of the optical disk. In addition, it is difficult for the heat mode super-resolution film to obtain sufficient stability during repeated operation due to the thermal fatigue in reproduction and recording.
In the photon mode, a photochromic material is employed as a super-resolution film, and an optical aperture or an optical mask is formed by utilizing coloring or decoloring of the photochromic material by irradiation of a reproduction beam. The photochromic material causes change in absorption characteristics by the phenomenon that an electron is excited from the ground level to an excited level of short life by irradiation of light and then the electron further shifts from the excited level to a metastable excited level of very long life where the electron is trapped. Accordingly, in order to perform repeated reproduction, the electron trapped at the metastable excited level is required to be deexcited to the ground level. However, an auxiliary beam is irradiated for deexcitation, which leads to two-beam operation, so that it is disadvantageous to realize a high-speed response. Moreover, change in transmittance of the photochromic material is brought about through a complicated process involving atomic migration or change in molecular bond, so that the stability in repeated operation is limited to about 10,000 times.
Japanese Patent Unexamined Publication No. 6-28713 discloses an optical disk provided with a shutter layer for stopping down the size of a light beam. The shutter layer is formed of a glass or resin matrix in which semiconductor particles are dispersed. Semiconductors described in the reference include CdS, CdSe, CdSSe, GaAs, a--Si, CdTe, CdSe, ZnO, ZnS, ZnSe, ZnTe, GaP, GaN, AlAs, AlP, AlSb and a--SiC. The reference describes that content of the semiconductor particles should preferably be 5 to 70 mol%, and particle size thereof should preferably be 0.1 to 50 nm. Described methods of forming the shutter layer include high-speed quenching and heat treatment, impregnation, a sol-gel method, spin-coating, sputtering and vacuum evaporation.
Likewise, Japanese Patent Unexamined Publication No. 6-44609 discloses an optical recording medium provided with an optical control film containing fine particles of semiconductor, metal or metallic compound, and having transmittance characteristics exhibiting a low transmittance to a low-intensity light beam while exhibiting a high transmittance to a high-intensity light beam. The optical control film is formed of a transparent dielectric, such as SiO.sub.2, Si.sub.3 N.sub.4, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, Li.sub.3 N, Ta.sub.2 O.sub.5 and Nb.sub.2 O.sub.3, or transparent resin in which semiconductor fine particles, such as CdS and CdSe, are dispersed. The reference describes that the particle size of the semiconductor fine particles should preferably be 1 to 20 nm. Described methods of forming the optical control film include sputtering, spin-coating and plasma CVD.
However, these references fail to describe a principle how the shutter layer or optical control film functions as a super-resolution film, and therefore the conditions for obtaining suitable properties to the super-resolution film are not obvious.
As described above, in order to realize super-resolution reproduction of an optical disk, it is required that change in transmittance of the super-resolution film should occur within a range of practical reproduction beam power; the magnitude of the transmittance change should be large enough; an optical aperture should be formed quickly within such a short period of time as the pass time of a reproduction beam spot; and stable repeated reproduction should be accomplished. However, no conventional super-resolution film can meet all of those requirements.
Meanwhile, in a recordable optical disk, a verifying operation of a recorded state is performed in recording. When a single-beam operated optical disk drive is used, after recording marks have been formed and then a period of time corresponding to one rotation of the optical disk has passed, a reproduction operation called a trial reproduction is performed so as to verify the recorded state. If an error is detected by the verifying operation, re-recording is performed at the same position where the initial recording was performed or at a different position.
Accordingly, even the ordinary recording operation without an error requires a period of time corresponding to two rotations of optical disk for recording and verifying. Further, when an error has occurred, it requires a period of time corresponding to three to four rotations of optical disk for recording, verifying and re-recording (and, if desired, re-verifying). Therefore, it is desired to shorten the time required for verifying.
Furthermore, when the super-resolution reproduction is applied to a medium that exhibits reflectance change of a recording layer in recording, like a phase change optical disk, it has been found that noises increase due to the characteristics of a super-resolution film. Therefore, it is necessary to compensate the noise level so as to improve the quality of reproduction signals.